Ion implantation has become the technology preferred by industry to dope semiconductors with impurities in the large-scale manufacture of integrated circuits. Ion dose is one of two important variables in defining a particular implant process (the other being ion energy, which determines implant depth). Ion dose relates to the concentration of implanted ions for a given area or volume of semiconductor material. Typically, high current implanters (generally greater than 1 milliamp (mA) ion beam current) are used for high dose implants, while medium current implanters (generally capable of up to about 1 mA beam current) are used for lower dose applications.
A conventional ion implanter comprises three sections or subsystems: (i) an ion source for outputting an ion beam, (ii) a beamline including a mass analysis magnet for mass resolving the ion beam, and (iii) a target chamber which contains the semiconductor wafer or other substrate to be implanted by the ion beam. Ion sources in ion implanters typically generate an ion beam by ionizing within a source chamber a source gas, a component of which is a desired dopant element, and extracting the ionized source gas in the form of an ion beam. The ion beam is directed along an evacuated beam path provided by the beamline. Energetic ions within the beam strike the substrate within the target chamber and are implanted therein. It is important in such an implantation system to insure that the implanted ions do not charge the surface of the wafer to such an extent that circuit components on the wafer are damaged. To prevent such detrimental effects of wafer charge accumulation, beam neutralization mechanisms are often implemented in an ion implanter. In addition, the energy of the implanted ions must be controlled in order to assure a uniform and desired energy distribution for the implant.
Plasma immersion ion implantation (PI-cubed or PI.sup.3) is an emerging technology wherein a substrate such as a wafer on a platen is immersed within a plasma in a chamber. Thus, the chamber functions as both the processing chamber and the plasma source. Typically, a voltage differential is periodically established between the walls of the chamber and the platen to attract ions in the plasma toward the substrate. A sufficient voltage differential will result in a pulsed ion implantation into the surface of the substrate.
As in conventional ion implantation systems, it is important to insure that wafer charging is minimized. In a PI-cubed system, wafer charging may be caused by either (i) accumulated charge from the implant pulses or (ii) exposure to the plasma in the chamber between implant pulses. In addition, as in conventional ion implantation systems, it is important to insure that only ions within a specific energy range are implanted into the wafer to yield a uniform implant energy distribution.
Accordingly, it is an object of the present invention to provide a system and method for maintaining a uniform implant energy distribution and for minimizing charge accumulation of a substrate implanted by a plasma immersion ion implantation system.